CA2091301A1

CA2091301A1 - Cooking vessel bottom and production method thereof

Info

Publication number: CA2091301A1
Application number: CA002091301A
Authority: CA
Inventors: Rino Gambini
Original assignee: TVS SpA
Current assignee: TVS SpA
Priority date: 1992-03-24
Filing date: 1993-03-09
Publication date: 1993-09-25
Also published as: GB2265543A; ITMI920683A0; ITMI920683A1; GB9305385D0; IT1254733B

Abstract

ABSTRACT OF THE DISCLOSURE
Flat cooking-vessel bottom whose outer surface is provided with equidistant concentric grooves in the form of circles and coated with two overlying layers of which the first is in contact with the metal and consists of a material with high radiation heat absorption capacity distributed uniformly over the entire bottom surface and the second layer consists of an enamel or silk-screen process paste resistant to abrasion and scratching distributed nonuniformly and preferably in segments and only on the lands between the surface grooves.

Description

2 ^ ~ l 3 0 ~

"Cooking vessel bottom and production method therefor"
* * * * *
BACKGROUND OF THE INVENTION
Field of the invention The present invention relates to a cooking vessel bottom. The pre~ent invention relates also to a production method for a cooking vessel bottom as well as vessels containing said bottom.
Description of rl t Cooking vessels such as saucepans, frying pans, pans, pie pans, pots and the like are generally made of aluminium or its alloys and have their bottom as flat as possible to ensure complete contact with the heat source.
During heating however the bottom surface is not heated uniformly~ Indeed, the bottom surface central zone is generally at a temperature relati~ely higher than that of the marginal zone. In addition, the lower face of the bottom in contact with the heat source is generally hotter that the top face~ These temperature differences are the source of different expansion of the metal, reducing the life of the vessel.
To obviate these shortcomings, from French patent no. 1,086.887 it is known to provide the bottom with concentric undulations or ~rooves which pe~mit absorption of the deformations due to radial forces created by irregular expansion of the different parts of the bottom.
one disadvantage of this type of bottom is that it is capable of absorbing from the hQat source cnly the thermal energy transmitted by direct contact, i.e. the contact heat. The heat transmitted by radi~tion is 3 ` ~i3~1 reflected from the bright surface of the aluminium or its alloys and thus contributes in reduced measure to heating of the vessel. In addition, since there is a téndency to produce vessel bottoms as thin as possible to save material, very often the flat shape of the bottom is deformed after long use of the vessel. This causes a radical drop in contact heat transmission.
To overcome this shortcoming, U.S. patent no.

4,926,843 describe~ a process in which a cooking vessel bottom provided with concentric and spaced grooves is coated with a layer of heat absorbent material only opposite the grooves while said layer is removed from the surface of the bottom in the zones between the grooves, i.e. in the concentric lands. The heat absorbing material is generally the same enamel used to coat the outer surface of the vessel.
Known enamels designed for coating aluminium or its alloys have the drawback of being soft, i.e.
presenting low abrasion and scoring resistance. This drawback is due to the composition of the frit of the enamel used, whose cooking temperature must be lower than the melting or softening temperature of the aluminium or its alloy. For this reason coating with enamels is limited to the side or internal surfaces (grooves) of the - 25 vessel while the surfaces in relief of the bottom remain bare. This limitatisn of th~ enamelled surfaces causes incomplete radiation heat absorption.
The purpose of the present invention is to solve the above mentioned shortcomings.
More particularly the purpose of the present 4 - ` ~ w ~

invention is to provide a cooking vessel bottom surface which would permit better heat transmission and, specifically, higher radiation heat absorption and stability in use.
SummaXy of the invention In accordance with a general aspect of the present invention these and other purposes are achieved by a cooking vessel bottom whose outer surface is provided with equidistant concentric grooves in circular form and is coated with two overlying layers of which the first, which is in contact with the metal, consists of a material having high radiation heat absorption capacity distributed uniformly over the entire bottom surface and the second layer consists of an enamel resistant to . abrasion and scratching distributed nonuniformly and preferab?y in segments and usually on the parts in relief of the surface between the grooves.
The cooking vessel is produced by ~onventional pressing cr turning of metal plates preferably of al~minium or its alloys. On the flat bottom surface of the vessel thus obtained are formed equidi.stant ~ircular concentric grooves by removal of bottom material performed with a lathe tool. For this purpose the vessel is fixed from the inside on a support and rotated around its central axis in relation to a punch having straight line movement.
Th~ dimensions of the grooves are not critical for the purposes of the present invention. Generally the depth of each groove is between 0.1mm and 2mm and the width is between 1F~ and 3mm. The surface in relief in 5 ~ 1 3 0 ~

ring form between the grooves has a width between lmm and 3mm. The distance between the centre lines of the grooves can vary between 4mm and 7mm.
The first layer of coating of ~he vessel is made with the same enamel generally used for coating the outer surface. The enamel can be any colour such as blue, gray, black, etc. Generally dark colours are preferred with matt or dull surface to ensure complete absorption of the radiation heat.
If the enamel which absorbs the heat is fluid it can be advantageously applied by spraying. Any material having high radiation heat absorption capacity can be used. These materials are well known in trade and can be porcelainized enamels or silicone lakes preferably with an alkaline silicate or borate base.
The second coating layer is preferably applied by silk-screen process depositing the silk-screen paste only on the concentric rings in relief and nonuniformly, passing it through a cloth screen with impression of the desired design.
The silk-screen paste can be ceramic-based when the first layer consists of porcelainized enamel or - silicone based when the first layer consists of silicone lake.
Any colour of silk-screen paste can b~ employed.
Generally a colour contrasting with that of the first coating layer is preferred so as to obtain aesthetic colour effeGts.
Examples of ceramic-base silk-screen paste which can be used for the bottom coating are those known on the 6 ~ 0 1 market under the names DA 710015 which is white, DA
210272 which is red, DA 110072 which is black and DA
710052 which is transparent, all manufactured and sold by the Bayer company.
Other colours of silk-screen paste can be obtained by mixing these and other known classical products for aluminium or aluminium alloys with colourings or pi~ments. Thus for example a dark gold ceramic-base silk-screen process can be obtained by mixing the base DA
71005 (transparent) of the Bayer company containing enamel frit with vitrified gold silicate base with mica coated with iron oxides known on the market under the name Mearlin Super Bronze R.
Any known type of silicone-base silk-screen process can be used. An illustrative example can be a modified polyester resin base silk-screen process manufactured and sold by the Tego Gold Schmidt company under the trademark HTFR containing dispersed mica coated with iron oxides known on the market under the trademark Mearlin Super Bronze R.
Brief Description of the Drawings The present invention will be better understood from the fsllowing detailed description of an example of a preferred embodiment with reference to the annexed drawings wherein:
FIG. 1 shows a partial plan view of the pan bottom in accordance with the present invention, FIG. 2 shows a schematic vlew of a cross section of FIG. 1 along plane of cut A A, FIG. 3 shows an enlarged schematic view of a part 13~1 of FIG. 2, FIG. 4 shows a schematic illustration of the pan forming phase, FIG. 5 shows a schematic illus-tration of the grooving phase of the bottom, FIG. 6 shows a schematic illustration of the spraying phase of the enamel of the first layer, and FIG. 7 shows a schematic phase of the silk-screen process phase.
Descxi~tion of the Preferred Embodiment With reference to figures 1-3 the cooking vessel ~1) as for example an aluminium or aluminium alloy pan comprises a flat bottom (2) and a side wall (3) bent inward. The external surface of the bsttom (2) of the - vessel (1) has $hree distinct surface parts, as follows:
- a flat central circular area (4) which is preferably concentric around the geometric centre of the vessel ( 1 ), - a generally annular area (5) provided with a plurality of grooves (6) and concentric circular lands (7), ~nd - a flat annular peripheral area (8) radiused to the side wall (3).
In accordance with the present invention the external surface of the bottom (2) is coated with two layers consisting of different enamels of which one layer (9) of an enamel having high radiation heat absorption capacity and the other (10~ of an enamel having high abrasion resistance.

The high radiation heat absorption capacity enamel is applied uniformly and in an unbroken manner over the entire bottom surface (2) including the central circular area (4), the grooves (6~, the lands (7) and the annular peripheral area (8) and the side wall (3)~
The layer (10) of high abrasion resistance enamel is applied only on the lands (7) and on the annular peripheral area (8) and in a broken manner, i.e. in segments, so that said lands (7) and said annular peripheral area (8) have portions not coated with said high abrasion-resistance enamel.
The parts coated with the layer (10) with high abrasion resistance represent the surface on which the vessel (1) rests.
The shape and dimensions of the segm~nts coated with ~he high abrasion-resistance enamel are not critical for the purposes of the present invention. Generally they are rectangular or square.
With reference to FIGS. 4 to 7 there is now described a preferred method of manufacture of the bottom ~2) of a cooking vessel (1) of aluminium or aluminium alloy of the present invention.
First, the cooking vessel Sl) is manufactured preferable by pressing of an aluminium plate (11~ in a press including a die ~12) and punch (13) having the form and dimensions of the desired vessel (1)~ The vessel (1) thus obtained is formed of a flat bottom (2~ and a sidP
wall (3) bent inward. In the internal part-of the vessel (1) is inserted a template (14) which xeceives the vessel and blocks it by a compressed air system. The template (14~ is connected to a motor (not illustrated) by a shaft 9 - 2~13~1 (15) so that said template and hence the vessel can be rotated around the axis of rotation of the motor..
At the external surface of the bottom (2) of the vessel (1) is the punch (16) of a lathe having straight-line movements of approach and withdrawal of said bottom(2) an~ advancement of the peripheral area (8) toward the central area (4). By approaching and pressing the punch (16) against the bottom (2) and rotating the template (14) and ~imultaneously moving said punch (16) toward the central area (4) there are executed a plurality of equidistant concentric unbroken grooves (6) in the annular area (5) of the bottom (2) by removal of material.
The bottom (2) having grooves (6) and lands (7) is ~hen sprayed over the entire external surface by a sprayer or nozzle (17) with a material (18) having high radiation heat absorption such as for example porcelainized enamel or a silicone lake as generally employed for coating aluminium or aluminium alloy Yessels. The spraying is done in a uniform and unbroken manner over the entire surface of the bottom (6l and the side wall (3).
The vessel (13 having the external surface of the bottom (2) and side wall (3~ coated uniformly and completely with a layer (9) of silicone lake or porcelainized enamel with high radiation heat absorption capacity is taken under the silk-screen process frame (19) consisting of a cloth screen with impression of the specific design which it is desired to obtain only on the lands (7), on the annular peripheral area (8) and lO ~ i301 optionally on the central part (4). The silk-screen process paste (20) is fed onto said frame (19) and spread by a rubber spatula (21).
The silk-screen process paste (20) filters only at S the holes in the cloth screen of the silk-screen process frame (19) impressing by a broken layer (10) and in segments only the parts of the bottom (2) corresponding to said holes. The thickness of said layer (10) is on the order of 5-20m~.
The recipient (1) is subsequently introduced in an oven with hot air circulation where it is baked at a temperature varying between 300C and 600C depending on the silk-screen process lake or enamel used.
The presence of the grooves (6), the coating with one layer (9) of material having high radiation heat absorption capacity and subsequent coating with a layer (10) of high abrasion resistance material provides excellent heat transmission from the source to the cooking vessel combined with high abrasion resistance, not common in these types of vesselsl and high dimensional stability even after prolonged use. The lands (7) between the grooves ~6) act as reinforcement ribs which help hold flat the bottom surface. In addition, both the surface of the lands (7) and tha~ of the grooves (6) absorb all kinds o~ heat, both ~ontact and radiation, and therefore the contact surface between the vessel and the heat sourc~ as well as the quantity of heat transmitted are enormously increased.
To these physical and technical characteristics and advantages are added unques~iGnable aesthetîc Y 1 3 0 ~

advantages due to the colour effect of the combination of colours of the two materials (18,20) used.
Although the invention has been described in conjunction with a detailed embodiment it is evident that many alternatives and variations will be apparent to those skilled in the art in light of the foregoing description. Accordingly, the invention is intended to embrace all of the alternatives and variations that fall within the spirit and scope of the appended claims.

Claims

1. Flat cooking-vessel bottom having the external surface provided with concentric grooves (6) and lands (7) in the form of equidistant circles and coated with two overlying layers of which one layer (9) is in contact with the metal and consists of an enamel or lake having high radiation heat absorption capacity and a second layer (10) consisting of an enamel having high abrasion and scoring resistance, said second layer (10) being applied only on the lands (7) and in a broken manner or in segments.

2. Flat cooking-vessel bottom in accordance with claim 1 characterized in that the external surface comprises three distinct surface parts, to wit:
- a flat central circular area (4), - an annular area (5) provided with grooves (6) and concentric lands (7) in the form of circles, and - an annular peripheral area (8).

3. Flat cooking-vessel bottom in accordance with claim 1 or 2 characterized in that the flat circular central area (4), the annular area (5), the annular peripheral area (8) and the side wall (3) are coated with a layer (9) of an enamel or lake having high radiation heat absorption capacity and only the lands (7) and the annular peripheral area (8) are coated in a broken or segmented manner by the second layer (10) consisting of an enamel having high abrasion and scoring resistance.

4. Flat cooking-vessel bottom in accordance with claim 1 characterized in that the segments coated with the enamel having high abrasion and scoring resistance have a rectangular or square form.

5. Flat cooking-vessel bottom in accordance with claim 1 characterized in that the enamel having high abrasion and scoring resistance is a silk-screen process paste with ceramic or silicone base.

6. Flat cooking-vessel bottom in accordance with claim 1 characterized in that the thickness of the second layer (103 is between 5mµ and 20mµ.

7. Aluminium or aluminium alloy cooking vessel having its flat bottom in accordance with claim 1.

8. Manufacturing method for a cooking vessel having a flat bottom in accordance with claim 1 and consisting of the following successive phases:
- manufacture of the vessel (1) by pressing or turning of an aluminium or aluminium alloy plate comprising a flat bottom (2) and a side wall (3) bent inward, - forming of the equidistant concentric circular grooves (6) on the external surface of the bottom (2) by removal of material with a lathe punch (16), - uniform application over the entire external surface of the bottom (2) and side wall (3) of a material (18) having high radiation heat absorption capacity, - application by silk-screen process in a broken manner and only on the lands of the bottom of silk-screen process paste (20) having high abrasion resistance which is spread with a spatula (21) on the silk-screen process frame (19) on the bottom (2) and consisting of a cloth screen with impression of the specific desired design, and - baking of the vessel (1) in a circulating air oven at a temperature between 300°C and 600°C.

9. Method in accordance with claim 8 characterized in that the grooves (6) are created in the annular area (5) of the bottom (2).

10. Method in accordance with claim 8 characterized in that the silk-screen process paste (20) has a ceramic base and the high heat radiation absorption capacity material (18) is porcelainized enamel.

11. Method in accordance with claim 8 characterized in that the silk-screen process paste (20) has a silicone base and the high heat radiation absorption capacity material (18) is silicone enamel.

12. Method in accordance with claim 8 characterized in that the silk-screen process paste (20) having high abrasion resistance is applied in a broken manner only on the lands (7) and the annular peripheral area (8).